Went out to fly my four packs on a warm (hot) morning last week. Didn't really rest the heli between flights.
Started with battery #4 but didn't fly because I had a sever tail wag all of a sudden, after hearing other stories I suspected a bearing had failed.

Replaced the torque tube bearing (used two on the rebuild) as it was notchy and noted that the wag had all but gone.
To be on the safe side I have also replaced the main bearings (found rough operation), the pinion bearing (rough) and the motor bearings (again rough).
Don't want to change the motor bearings again in a hurry as the motor's a pain to get out the model and the original bearings even harder to get out the motor.

While in service mode, checked both head and tail thrust bearings. The head was nearly dry and the tail very dry, re greased.

Heli is standard except mini vbar, I don't fly 3d and I log my flights. So after 40 flights and 3.5 hrs flying quite a few bearings needed replacing. My 500 lasted much longer but I guess the 600 bearings are under greater force. Interested to see how long RCBearings last.